The utility of a crosslinked polymer gel as a permeability reducing agent in or near a subterranean hydrocarbon-bearing formation to facilitate hydrocarbon recovery from the formation has long been known in the art. U.S. Pat. No. 3,762,476 to Gall is representative of conventional teaching in the art of its time. Gall discloses a conformance improvement treatment process, wherein a permeability reducing crosslinked polymer gel is formed in situ by injecting an aqueous slug containing a crosslinkable water-soluble polymer into a subterranean treatment region followed in sequence by an aqueous slug containing a crosslinking agent including a polyvalent metal cation. Useful polyvalent metal cations listed in Gall include iron(II), iron(III), aluminum(III), chromium(III), calcium(II), and magnesium(II).
Sequential injection of the gel components as separate and distinct slugs into the treatment region is essential to the teaching of treatment processes such as Gall insofar as surface contacting of the polymer and crosslinking agent was believed to cause premature gelation of the gel components prior to reaching the treatment region. If prematurely formed at the surface or in the well bore, placement in the treatment region of the gels taught by Gall was difficult, if not impossible, to achieve. Consequently, sequential injection of the gel components in concept avoided premature gelation by delaying contacting of the components until they were displaced out into the treatment region. In situ contacting of the polymer and crosslinking agent as required by Gall, nevertheless, proved operationally unattractive in many hydrocarbon recovery applications because of the difficulty in achieving adequate mixing of the gel components in situ. Without adequate mixing, the gels of Gall were poorly formed, resulting in weak and unstable gels which performed ineffectively as permeability reducing agents.
In response to the shortcomings of sequential injection processes such as Gall, U.S. Pat. No. 4,683,949 to Sydansk et al identified specific gel components and gelation parameters for a crosslinked polymer gel having utility in a conformance improvement treatment process, whereby the polymer and crosslinking agent of the gel could be mixed at the surface in a homogeneous gelation solution and placed in the treatment region by injection therein as a single slug. Thus, Sydansk et al overcame the inherent operational limitations of processes such as Gall which required sequential injection and in situ mixing of the gel components. The gel technology of Sydansk et al was predicated on the finding that a chromium(III)/carboxylate complex crosslinking agent could be mixed with a crosslinkable polymer at the surface to form a gelation solution which produced a uniquely stable, highly predictable and beneficially performing gel in situ.
Although the process taught by Sydansk et al is generally effective for most treatment applications, certain treatment applications have proven problematic. For example, high temperature regions having a substantial concentration of carbonate minerals in the rock matrix are a difficult environment for treatment. The carbonate minerals partially dissolve into the formation waters of the high temperature treatment region and tend to damage the gel or gelation solution on contact. The carbonate minerals form alkaline species in solution, attacking or interfering with the chromium(III) which is present in the crosslinking agent. The alkaline species tend to convert chromium(III) to chromic hydroxide precipitate, depleting the availability of chromium(III) for polymer crosslinking. Consequently, the stability and effectiveness of the resulting gel as a permeability reducing agent can be significantly diminished. As such, a need exists for a process employing a crosslinked polymer gel as a permeability or fluid mobility reducing agent, wherein the gel has improved stability and effectiveness in alkaline environments, and particularly in high temperature carbonate treatment regions.
Accordingly, it is an object of the present invention to provide a process for applying a stabilized crosslinked polymer gel to a subterranean region which is highly alkaline. It is more particularly an object of the present invention to reduce the permeability of or fluid mobility within the alkaline treatment region to which the stabilized gel is applied. It is also an object of the present invention to improve the recovery of hydrocarbons from a hydrocarbon-bearing formation which contains alkaline treatment region or which is in fluid communication with the alkaline treatment region. More particularly, it is an object of the present invention to improve the recovery of hydrocarbons from a hydrocarbon-bearing formation which contains a carbonate treatment region or which is in fluid communication with a carbonate treatment region. It is another object of the present invention to improve the recovery of hydrocarbons from a hydrocarbon-bearing formation which contains a high temperature treatment region or which is in fluid communication with the high temperature treatment region. These objects and others are achieved in accordance with the invention described hereafter.